In a drill string, the drill bit may be turned by a mud motor, especially when directional drilling using a rotary steerable system (RSS). Mud motors, as understood in the art, often utilize Moineau or progressive cavity pumps. Progressive cavity pumps include a multi-lobed rotor which eccentrically rotates within a correspondingly lobed stator. In order to mechanically couple the eccentrically rotating rotor to a concentrically rotating drive shaft, a transmission assembly may be utilized. In some cases, a flexible shaft, known as a flexshaft, may be utilized. In other cases, a rigid transmission shaft which is mechanically coupled to the rotor and drive shaft by one or more joints may be utilized. However, typical joints create point or line stress loading forces therein, contributing to wear and excessive stresses on the drives.